Fatigue Life Study for Floorbeam Cutout Using the Theory of Critical Distances

Orthotropic steel bridge deck is widely used in civil engineering because of its light weight, high ultimate bearing capacity and wide application range. But its fatigue problem is serious. Steel box girder of diaphragm plate arc incision anti-fatigue design is still a difficulty, to effectively predict the fatigue life of the typical fatigue vulnerability details, using ABAQUS finite element analysis software, set up three kinds of fatigue specimen the simplified finite element model of arc incision, through three different radius arc incision specimen under tensile load of 50 kN finite element model, the independent stress nephograms were obtained respectively. It can be seen that the first principal stress at the notch decreases with the increase of the notch radius. When the radius increases from 10 mm to 20 mm, the first principal stress decreases from 319.8 MPa to 253.7 MPa, with a reduction of 66.1 MPa. When the radius increases from 20 mm to 30 mm, the first principal stress decreases from 253.7 MPa to 225.6 MPa, with a decrease of 28.1 MPa, which becomes gentle compared with before. Then the critical distance theory is introduced to analyze the characteristic stress in the stress concentration area of curved notch, and the point method and line method are used to calculate the characteristic stress. ABAQUS was used to set the path from the maximum principal stress peak according to the direction of the highest stress gradient to extract the first principal stress at each point and the distance between the point and the maximum principal stress peak. On the hot spot path, the maximum principal stress is inversely proportional to the distance. Within the critical distance of 1.2 times, the smaller the notch radius, the higher the stress level; In the region beyond 2 times the critical distance, the maximum principal stress curves of the notch radius of 20 mm and 30 mm show a close trend, indicating that the notch is no longer the key factor affecting the stress level. Thus, key parameters such as the critical distance and the threshold value of crack growth are obtained. Combined with the results of fatigue limit and FE-safe life, the fatigue life prediction models of three notched members were established. The accuracy of the simplified model was verified by the fatigue tests of the diaphragmcurved notch segments. The influence of different notch radius on the fatigue life of the specimens was studied.
The results show that:1) the predicted stress value based on the point method is 7%~13% higher than that based on the line method, and the predicted fatigue life value based on the point method is more than 50% lower than that based on the line method at the arc notch of the steel box girder diaphragm. The point method is more conservative than the line method; 2) the fatigue prediction model of the steel box girder diaphragm arc notch proposed in this paper has a high accuracy, and the error between it and the fatigue test results of the section model is basically within the range of 20%; 3) no matter using point method or line method, the calculated characteristic stress decreases with the increase of notch radius, the fatigue life increases with the increase of notch radius, the fatigue life of steel box girders can be improved by increasing the radius of the curved notch; 4) the fatigue specimens used in this paper are all smooth curved notch specimens. For the components with initial defects, the calculation model of fatigue life evaluation using critical distance theory needs to be further studied.